1. Field of the Invention
The present invention relates to an oxide superconductor and a method of preparing the same, and more particularly, it relates to an oxide superconductor having a high superconductivity transition temperature (Tc=134 K.) exceeding the liquid nitrogen temperature and a method of preparing the same.
2. Description of the Background Art
A superconductor has unique properties of its own such as zero electrical resistance, complete diamagnetism and a Josephson effect. Due to such properties, wide applications of superconductors are expected in relation to power transportation, a generator, nuclear fusion plasma confinement, a magnetic levitation train, a magnetic shield, a high-speed computer and the like.
In 1986, Bednorz and Muller discovered an oxide superconductor (La.sub.1-X Ba.sub.X).sub.2 CuO.sub.4 having a high superconductivity transition temperature Tc of about 30 K. Following this discovery, there have been reported oxide superconductors having high superconductivity transition temperatures Tc such as YBa.sub.2 Cu.sub.3 O.sub.7 (Tc=90 K.), Bi-Sr-Ca-Cu-O (Tc=110 K.) and Tl-Ba-Ca-Cu-O (Tc=125 K.). Many researches are now being made in relation to methods of preparing such superconductors, physical properties and applications thereof and the like.
In the process of such researches, a superconductor having a superconductivity transition temperature Tc of 133.5 K. has recently been confirmed in relation to Hg-Ba-Ca-Cu-O ceramics (A. Schilling et al., Nature 363 (1993) 56).
As to the superconductor exhibiting a superconductivity transition temperature Tc of 133.5 K., it is inferred from observation of its lattice image with a transmission electron microscopy that a substance expressed in a chemical formula HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.Y (Y: arbitrary number) having such a crystal structure that eight atomic layers of HgO.sub.Z -BaO-CuO.sub.2 -Ca-CuO.sub.2 -Ca-CuO.sub.2 -BaO are stacked exhibits the high superconductivity transition temperature Tc. However, no crystal structure of a superconducting crystal phase has been clarified with respect to this superconductor.
A. Schilling et al. further reported that they prepared the superconductor by preparing Ba.sub.2 CaCu.sub.2 O.sub.Y as a precursor, mixing the same with HgO, vacuum-sealing the mixture in a silica tube, introducing the silica tube into a steel container and firing the same at 800.degree. C. for 5 hours. However, they clarified no mixing ratio of the precursor to HgO. Further, they reported that the silica tube which was introduced into the steel container was cracked after firing, and it was difficult to reproduce their experiment.
Although the superconductor having a superconductivity transition temperature Tc of 133.5 K. was confirmed in the aforementioned manner, it has generally been extremely difficult to reproduce equivalent superconductivity since there has been established no method of preparing the same.